Fuel cell with randomly-dispersed carbon fibers in a backing layer

ABSTRACT

A fuel cell ( 40 ) includes first and second catalysts ( 12 ′), ( 14 ′) secured to opposed surfaces of an electrolyte ( 16 ′); a first flow field ( 26 ′) secured in fluid communication with the first catalyst ( 12 ′) defining a plurality of flow channels ( 30 A′,  30 B′,  30 C′,  30 D′) between a plurality of ribs ( 32 A′,  32 B′,  32 C′,  32 D′,  32 E′) of the first flow field ( 26 ′); and a backing layer ( 42 ) secured between the first flow field ( 26 ′) and the first catalyst ( 12 ′). The backing layer ( 42 ) includes a carbon black, a hydrophobic polymer, and randomly-dispersed carbon fibers ( 44 ). The carbon fibers ( 44 ) are at least twice as long as a width (46) of the flow channels ( 30 A′,  30 B′,  30 C′,  30 D′) defined in the adjacent first flow field ( 26 ′). The backing layer ( 42 ) replaces a known substrate ( 22 ) and diffusion layer ( 18 ).

TECHNICAL FIELD

The present invention relates to fuel cells that are suited for usage intransportation vehicles, portable power plants, or as stationary powerplants, and the invention especially relates to a fuel cell having abacking layer secured between a flow field and a catalyst.

BACKGROUND ART

Fuel cells are well-known and are commonly used to produce electricalenergy from reducing and oxidizing reactant fluids to power electricalapparatus, such as apparatus on-board space vehicles, transportationvehicles, or as on-site generators for buildings. Each individual fuelcell generally includes an anode catalyst and a cathode catalystseparated by an electrolyte, such as a proton exchange membrane (“PEM”)as known in the art. Frequently, a diffusion layer is secured betweenthe catalyst and a substrate layer. The substrate layer is usuallysecured between the diffusion layer and a flow field. If there is nodiffusion layer, the substrate is secured between the catalyst and theflow field. Flow fields define flow channels for directing reactantstreams through the fuel cell in fluid communication through thediffusion and substrate layers with the catalysts. As is known, the flowfields may be porous water transport plates or solid separator plates.

The diffusion layer is typically a highly porous, electrical conductormade from carbon black and a hydrophobic polymer, such aspolytetrafluoroethylene. Depending on the fuel cell design, thediffusion layer is usually hydrophobic, however, the diffusion layer maybe partially hydrophobic and partially hydrophilic to facilitatesimultaneous liquid and gaseous transport through the layer. Thediffusion layer is usually about 25-100 microns thick. The diffusionlayer facilitates transfer of the reactant streams through the fuel cellby minimizing the thickness of water films on the surface of thecatalysts. The diffusion layer also facilitates the removal of productwater from the fuel cell.

The substrate layer adjacent the diffusion layer is highly porous andmade from expensive carbon fibers and a well known manufacturing processthat requires high temperature graphitizing. Depending on the celldesign, the substrate layer may be either hydrophobic or hydrophilic.The substrate is usually about 150-300 microns thick. The substratefacilitates the transport of reactant streams, water vapor, liquid waterand electrons. The substrate conducts electrons both through the planeof the substrate and in the plane of the substrate from a centerline ofan adjacent flow channel to ribs of the flow field, wherein the flowchannel is defined between ribs of the flow field. The substrate alsofacilitates the diffusion and flow of reactant streams and product waterboth through the plane of the substrate and in the plane of thesubstrate from the flow channel to the centerline of the flow channelbeneath the rib of the flow field, or vice versa. Also, the substratemust have a flexural strength adequate to distribute an axial pressureload relatively uniformly over total surface areas of adjacent layers.For example, the pressure load must be distributed evenly across theflow channels of the flow field to prevent the substrate from deforminginto the flow channels. An exemplary flexural strength is about 200kilogram force per square centimeter (“kgf /cm²”). U.S. Pat. No.4,851,304 to Miwa et al. describes properties of typical fuel cellsubstrates. Planar types of fuel cells are secured in compression in afuel cell stack by a combination of pressure plates and tie-rods, as iswell known. This axial compressive force minimizes the resistance of thecells and is required to obtain suitable fluid seals. The substrate musthave a compressive strength that is typically at least two times theaxial force on the cell stack. Typical compressive strength of a priorart substrate is greater than 10 kgf/cm².

In fuel cells of the prior art, it is known that carbon or graphitelayers or papers are traditionally secured between the catalysts andflow fields as diffusion and/or substrate layers. However, the use ofknown carbon or graphite layers or papers presents significant problems,including high manufacturing costs, impeding the diffusion of hydrogenand oxygen through pores defined by the layers, and impeding the outflowof fuel cell product water from a cathode catalyst.

A partial solution to the use of carbon or graphite paper is disclosedin U.S. Pat. No. 5,707,755, entitled “PEM/SPE Fuel Cell” that issued onJan. 13, 1998 to Grot. The patent discloses, instead of the carbon orgraphite layers or papers, the use of a plurality of electricallyconductive filaments secured with a specific orientation with respect toflow channels, or grooves of a flow field. The specific orientation islongitudinal so that the filaments extend across, and do not fall into,the flow channels, or grooves, that direct reactant flow through thefuel cell.

Nonetheless, the prior art has limitations. Manufacturing a fuel cellwith conductive filaments having such a specific orientation imposessubstantial cost and manufacturing burdens. Moreover, the prior artfails to rectify the significant substrate manufacturing costsassociated with the mass production of fuel cells. The substrates arecostly because they include expensive carbon fibers and are manufacturedthrough a costly high temperature graphitizing process well known in theart. Accordingly, there is a need for a fuel cell that minimizessubstrate costs by replacing the prior art substrates with a costeffective material.

DISCLOSURE OF INVENTION

The invention is a fuel cell with randomly-dispersed carbon fibers in abacking layer of the fuel cell. The fuel cell produces electricity fromreducing fluid and oxygen containing oxidant reactant streams, andcomprises first and second catalysts secured to opposed surfaces of anelectrolyte, such as a proton exchange membrane (“PEM”) well known inthe art. A first flow field is secured in fluid communication with thefirst catalyst, the first flow field defining a plurality of flowchannels between a plurality of ribs of the first flow field. Thebacking layer is secured between and in fluid communication with thefirst flow field and the first catalyst so that the backing layer is atleast coextensive with the first flow field and the first catalyst. Thebacking layer is made from between 5 weight percent (“wt%”) and 25 wt %carbon black, between 50 wt % and 90 wt % carbon fibers, and between 5wt % and 25 wt % of a hydrophobic polymer. For purposes herein, a“hydrophobic polymer” is defined as a polymer having a surface energythat is less than 40 dynes/cm². Examples of hydrophobic polymers knownin the art are polytetrafluoroethylene and polyvinyldene fluoride. Thecarbon fibers are randomly dispersed with the carbon black andhydrophobic polymer so that the backing layer, which is bonded to thefirst catalyst layer and secured between the first catalyst and thefirst flow field, has a thickness between the first catalyst and thefirst flow field of between about 25 and about 250 microns, and acompressive strength greater than 5 Kgf/cm². Because the backing layerreplaces the costly substrate and diffusion layers, fuel cellmanufacturing costs and requirements are minimized.

In a preferred embodiment, the first catalyst is an anode catalyst andthe fuel cell includes a diffusion layer and substrate or substratelayer between the second catalyst and second flow field. The inventionincludes a method of manufacturing the backing layer including the stepsof: (a) dispersing the carbon black, carbon fibers, and hydrophobicpolymers in an aqueous suspension, (b) removing the water by filtration;(c) thermally processing the constituents of the layer to melt or curethe hydrophobic polymer. The layer may then be bonded to the anodecatalyst by known means.

Accordingly, it is a general purpose of the present invention to providea fuel cell with randomly-dispersed carbon fibers in a backing layerthat overcomes deficiencies of the prior art.

It is a more specific purpose to provide a fuel cell withrandomly-dispersed carbon fibers in a backing layer that provides a costeffective fuel cell by substituting an anode backing layer for substrateand diffusion layers.

These and other purposes and advantages of the present fuel cell withrandomly-dispersed carbon fibers in a backing layer will become morereadily apparent when the following description is read in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a simplified schematic representation of a prior art fuelcell.

FIG. 2 is a simplified schematic representation of a preferredembodiment of a fuel cell with randomly-dispersed carbon fibers in abacking layer constructed in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings in detail, a typical prior art fuel cell isshown in FIG. 1 and is designated by the reference numeral 10. As iswell known in the art, the fuel cell 10 includes a first catalyst 12,such as an anode catalyst, and a second catalyst 14, such as a cathodecatalyst, secured to opposed surfaces of an electrolyte 16, such as aproton exchange membrane (“PEM”). The fuel cell 10 also includes ananode diffusion layer 18 secured to the first catalyst 12 and a cathodediffusion layer 20 secured to the second catalyst 14; an anode substrate22 or substrate layer secured to the anode diffusion layer 18 and acathode substrate 24 secured to the cathode diffusion layer 20; a firstflow field 26 secured in fluid communication with the first catalyst 12;and, a second flow field 28 secured in fluid communication with thesecond catalyst 14.

The first flow field 26 defines a plurality of flow channels 30A, 30B,30C, 30D between a plurality of ribs 32A, 32B, 32C, 32D, 32E of thefirst flow field 26. The second flow field 28 defines a plurality offlow channels 31A, 31B, 31C, 31D between a plurality of ribs 33A, 33B,33C, 33D, 33E of the flow field 28. The operation of such a prior artfuel cell 10 is well known in the art as disclosed in U.S. Pat. No.6,322,915, entitled “Humidification System For A Fuel Cell Power Plant”that issued on Nov. 27, 2001 to Collins et al., which patent is owned bythe owner of all rights in the present invention.

FIG. 2 shows the present invention of a fuel cell withrandomly-dispersed carbon fibers in a backing layer and is generallydesignated by the reference numeral 40. For purposes of efficiency,those components of the present invention that are virtually the same ascomparable elements as the prior art fuel cell 10 described above andshown in FIG. 1 are shown in FIG. 2 having a prime of the same referencenumeral shown in FIG. 1. For example, the proton exchange membrane 16shown in FIG. 1 is designated by the reference numeral 16′ in FIG. 2.

The present invention of a fuel cell with randomly-dispersed carbonfibers in a backing layer includes a first catalyst 12′, such as ananode catalyst, and a second catalyst 14′, such as a cathode catalyst,secured to opposed surfaces of an electrolyte 16′, such as a “PEM”. Thefuel cell 40 also includes a cathode diffusion layer 20′ secured to thesecond catalyst 14′; a cathode substrate 24′ secured to the cathodediffusion layer 20′; a second flow field 28′ secured in fluidcommunication with the second catalyst 14′; and, a first flow field 26′secured in fluid communication with the first catalyst 12′. The firstflow field 26′ defines a plurality of flow channels 30A′, 30B′, 30C′,30D′ between a plurality of ribs 32A′, 32B′, 32C′, 32D′, 32E′ of thefirst flow field 26′.

The fuel cell 40 also includes a backing layer 42 secured between and influid communication with the first catalyst 12′ and the first flow field26′. The backing layer 42 replaces the anode diffusion layer 18 andanode substrate 22 of the prior art fuel cell 10 as shown in FIG. 1. Thebacking layer 42 is made from between 5 weight percent (“wt%”) and 25 wt% carbon black, between 50 wt % and 90 wt % carbon fibers, and between 5wt % and 25 wt % of a hydrophobic polymer. The carbon fibers 44 have alength that is at least twice as long as a width 46 of a widest flowchannel 30D′, wherein the width 46 of the widest flow channel 30D′ is ashortest distance between flow field ribs 32D′, 32E′ defining the widestflow channel 30D′ of the first flow field 26′. The carbon fibers 44 arerandomly dispersed with the carbon black and hydrophobic polymer withinthe backing layer 42, resulting in the layer 42 having a thicknessbetween about 25-250 microns thick, wherein the thickness is a shortestdistance through the layer between the first catalyst 12′ and the firstflow field 26′. The layer 42 also has a compressive strength of greaterthan 5 Kg/cm².

Exemplary carbon blacks appropriate for use in the backing layer 42include a carbon black available under the product name “VULCAN XC-72”from the Cabot Corporation of Billerica, Mass., U.S.A. A suitablehydrophobic polymer is polytetrafluoroethylene (“PTFE”) available underthe product name “PTFE Grace 30” from the E.I. du Pont de Nemours andCompany of Wilmington, Del., U.S.A. Other acceptable hydrophobicpolymers are known in the art. A suitable carbon fiber is availableunder the product name “SIGRAFIL C” from the SGL Carbon Group company ofSt. Marys, Pa., U.S.A. A family of high strength carbon fibers is alsoavailable from the BP Amoco Performance Products company of WarrensvilleHeights, Ohio, U.S.A.

FIG. 2 shows the fuel cell 40 having parallel flow channels of similaror identical widths, and wherein the carbon fibers have a length that isat least twice as long as the width 46 of flow channel 30D′. While FIG.2 shows the fuel cell 40 having parallel first flow channels 30A′, 30B′,30C′, 30D′, other exemplary flow channels, such as serpentine channels,voids of various dimensions, or voids in a “checkerboard” arrangementaround square-shaped flow field ribs, may be used as known in the art.The operation of a fuel cell with flow channels in such a “checkerboard”arrangement is well known as disclosed in U.S. Pat. No. 5,503,944, thatissued on Apr. 2, 1996 to Meyer et al., which patent is owned by theowner of all rights in the present invention. For purposes of thepresent invention where flow channels (not shown) have dissimilarwidths, the length of the carbon fibers 44 would be at least twice aslong as a width of a widest flow channel (not shown) of dissimilar widthflow channels.

The carbon fibers 44 serve to enhance electrical conductivity within thefuel cell 40 and to contribute to the backing layer 42 having a minimalthickness with an adequate flexural and compressive strength so that thelayer 42 can withstand an axial pressure load and prevent the layer 42from deforming into flow channels 30A′, 30B′, 30C′, 30D′.

As is known, prior art fuel cells generate water at a cathode catalystas fuel cell product water. If the product water is not efficientlyremoved from the cathode catalyst, the product water will accumulateadjacent the cathode catalyst effectively “flooding” the catalyst. By“flooding” the catalyst, it is meant that a gaseous oxidant reactantstream cannot efficiently flow or diffuse into contact with the cathodecatalyst. Because the oxidant cannot contact the flooded cathodecatalyst, performance of the fuel cell is degraded. Therefore, in orderto efficiently remove fuel cell product water, known cathode diffusionlayer 20 and cathode substrate layer 24 are dimensioned to definespecific ranges of pore sizes and porosities, or percent pore volumes.However, such efficient movement of fuel cell product water is not arequirement of the prior art fuel cell 10 anode diffusion layer 18 oranode substrate layer 22.

Consequently, in a preferred embodiment of the present invention, thebacking layer 42 is only secured adjacent the anode catalyst 12′, asshown in FIG. 2. The backing layer 42 secured adjacent the anodecatalyst 12′ may therefore be thinner and more hydrophilic than theprior art anode substrate 22 that is secured adjacent to the anodecatalyst 12′ of the prior art fuel cell 10. Hence, a preferredembodiment of the invention is the fuel cell 40 with the backing layer42 secured adjacent to the anode catalyst 12′. However, alternativeembodiments of the invention include a fuel cell 40 with backing layers42 secured adjacent to both the anode and cathode catalysts 12′, 14′, orsecured adjacent to only the cathode catalyst 14′.

The invention includes a method of manufacturing the backing layer 42including the steps of: (a) randomly dispersing the carbon black, carbonfibers 44, and hydrophobic polymer in an aqueous suspension, (b) thenremoving the water from the aqueous suspension by filtration or otherwater removal methods known in the art, (c) and then thermallyprocessing the carbon black, carbon fibers 44 and hydrophobic polymerconstituents of the layer to melt the hydrophobic polymer. The thermalprocessing is normally done in air for 5-15 minutes at a temperatureequal to or within plus or minus 15 degrees centigrade of the meltingpoint or cure point of the hydrophobic polymer. The resulting backinglayer may then be bonded to an anode catalyst by known means.

Where the invention is the FIG. 2 embodiment having the backing layer 42secured only adjacent the second catalyst 12′, the fuel cell 40 may befabricated as follows. First, the cathode diffusion layer 20′ is appliedto the cathode substrate 24′ by known means. Second, anode and cathodecatalysts 12′, 14′ are applied to an electrolyte 16′, such as a “PEM” toform a membrane electrode assembly (“MEA”). Then the “MEA” is bonded toa cathode diffusion layer 20′. Finally, the backing layer 42, made asdescribed above, is bonded to the anode catalyst 12′ by known means.

The patents referred to above are hereby incorporated herein byreference.

While the present invention has been described and illustrated withrespect to a particular construction of a fuel cell withrandomly-dispersed carbon fibers in a backing layer it is to beunderstood that the invention is not to be limited to the described andillustrated embodiments. Accordingly, reference should be made primarilyto the following claims rather than the foregoing description todetermine the scope of the invention.

1. A fuel cell (40) for producing electricity from reducing fluid andoxygen containing oxidant reactant streams, the fuel cell (40)comprising: a. a first catalyst (12′) and a second catalyst (14′)secured to opposed surfaces of an electrolyte (16′); b. a first flowfield (26′) secured in fluid communication with the first catalyst(12′), the first flow field (26′) defining a plurality of flow channels(30A′, 30B′, 30C′, 30D′) between a plurality of ribs (32A′, 32B′, 32C′,32D′, 32E′) of the first flow field (26′); c. a backing layer (42)secured between and in fluid communication with the first flow field(26′) and the first catalyst (12′); and, d. wherein, the backing layer(42) includes: i. between 5 wt % and 25 wt % carbon black; ii. between50 wt % and 90 wt % carbon fibers (44), where the carbon fibers (44)have a length that is at least twice as long as a width (46) of a widestflow channel (30′D), wherein the width (46) of the widest flow channel(30′D) is a shortest distance between flow field ribs (32D′, 32E′)defining the widest flow channel (30D′) of the first flow field (26′);iii. between 5 wt % and 25 wt % of a hydrophobic polymer; and, iv. thecarbon fibers (44) being randomly dispersed with the carbon black andhydrophobic polymer so that the resulting backing layer (42) has athickness between the first catalyst (12′) and the first flow field(26′) of between about 25 and 250 microns, and a compressive strengthgreater than 5 Kgf/cm².
 2. The fuel cell (40) of claim 1, wherein thefirst catalyst (12′) is an anode catalyst and the second catalyst (14′)is a cathode catalyst secured to a cathode diffusion layer (20′) that issecured to a cathode substrate (24′).
 3. The fuel cell (40) of claim 1,further comprising a second backing layer secured between the secondcatalyst (14′) and second flow field (28′).
 4. The fuel cell (40) ofclaim 1 wherein the first catalyst (12′) is a cathode catalyst and thesecond catalyst (14′) is an anode catalyst secured to an anode diffusionlayer (20′) that is secured to an anode substrate (24′).
 5. The fuelcell (40) of claim 1, wherein the electrolyte (16′) is a proton exchangemembrane.
 6. A method of manufacturing a backing layer (42) for useadjacent a catalyst (12′, 14′) in a fuel cell (40), the methodcomprising the steps of: a. randomly dispersing a carbon black, carbonfibers (44), and a hydrophobic polymer in an aqueous suspension, whereinthe carbon fibers (44) have a length that is at least twice as long as awidth (46) of a widest flow channel (30′D) of a flow field (26′) securedin fluid communication with the catalysts (12′, 14′); b. removing thewater from the aqueous suspension; c. thermally processing the carbonblack, carbon fibers (44), and hydrophobic polymer of the layer (42) tomelt the hydrophobic polymer.
 7. A method of manufacturing a fuel cell(40) comprising the steps of: a. securing a cathode diffusion layer(20′) to a cathode substrate (24′); b. securing an anode catalyst (12′)and a cathode catalyst (14′) to opposed surfaces of an electrolyte (16′)to form a membrane electrode assembly; c. bonding the cathode catalyst(14′) to the cathode diffusion layer (20′); and, d. bonding the backinglayer (42) of claim 1 to the anode catalyst (12′).